Based on my understanding, due to the nature of the App.config file it's not possible to reference a constant defined in your application inside the aforementioned file. This can be seen in the
Modularity QuickStart, which define the module names using constants in the
WellKnownModuleNames static class but needs to write the corresponding module names in the
App.config file.

As this is not strictly related to Prism, I believe you can find more information about the
App.config file and better support regarding this topic as well in the
WPF forums: